Government agencies are assessing the legality of a diet that promises people they will lose up to half a kilogram a day by using a homeopathic fertility hormone banned in the United States.
The diet, which is gaining popularity in New Zealand, involves taking a substance known as hCG and restricting food to 500 calories a day for up to 40 days.
A nutritionist has warned that this is far below the amount of food people need to function.
The diet is based on the concept that hCG - a hormone produced in placentas and found in pregnant women's urine - can absorb excess fat tissue.
Many companies selling the product also claim it can curb appetite and speed up the metabolism.
But numerous studies have found no scientific evidence that the hormone causes weight loss.
In the US, the hormone is approved by the Food and Drug Administration (FDA) only as a prescription injection drug to treat infertility and some other conditions.
This week, the Commerce Commission received a complaint about one of the products, alleging misleading claims, said spokeswoman Allanah Kalafatelis.
The commission is assessing the complaint and will decide next week whether to investigate further.
The Ministry of Health is also looking into the sale of hCG diet drops to ensure it is within the law governing sales of medicines.
Medsafe compliance manager Derek Fitzgerald, said some homoeopathic remedies contained so little of the active ingredient that they were not regarded as being any risk.
But making a product look as if it contained a prescription-only medicine, or making therapeutic claims about a product, could still put it outside the law.
Mr Fitzgerald said the FDA had warned companies selling homeopathic products that the sales and claims were illegal in the US.
The owner of Manawatu-based HCG Diet Solutions, who identified herself only as Sarah, said interest in the drops had "exploded" in the past 18 months, and she had sold "thousands".
She said the diet was completely safe, as long as people did not use it for more than 40 days without a six-week break. Active people could increase their food intake to 800-1000 calories a day.
"It's brilliant - it really helps people," she said.
Sarah sells a 23-day supply of hCG - human chorionic gonadotropin - for $135 and a 40-day supply for $180. Her cheapest option is an eight- to 10-day supply for $55.
The diet has three phases: "loading", "days 3-21 or 42" and "maintenance".
The first phase is two days of eating a very high fat diet and taking hCG drops three times a day - this is supposed to reset the body's fat-regulating system.
Dieters then cut back to 500 calories a day for 19 or 40 days while continuing the hCG drops.
By the maintenance phase, dieters have completed the course of hCG and slowly increase their calorie intake to "stabilise" their new weight.
The managing director of Nova Nutrition, Catherine Sissons, said any diet that eliminated food groups meant users would be less likely to be nourishing their bodies correctly.
The body required a certain level of nutrients to maintain the immune system and for the metabolism to operate efficiently.
What it is
• The diet has three phases: loading, days 3 to 21 or 42, and maintenance.
• Dieters take three doses of hCG each day. The hormones come in dropper bottles and users keep the liquid under their tongue for 15 seconds.
• No food or water is allowed 15 minutes before or after the drops are taken.
• Only 500 calories can be consumed each day.
• Sugar, fat and oil are banned.